The present invention relates to a vacuum brake booster for an automotive vehicle comprising a vacuum casing having a movable wall therein and a tube connecting both end walls of the casing. The movable wall divides the vacuum casing into a vacuum chamber and a working chamber, the pressure in the working chamber being controllable by a valve, and a force which is proportional to the differential of pressure in the two chambers acts on the movable wall which is sealed and slidable relative to the tube and mechanically connected with the valve housing arranged within the tube by cross members guided in axial slots of the tube.
Brake booster for automotive vehicles wherein the differential of pressure between a vacuum and atmospheric pressure produces an additional force are known. The brake booster comprises a vacuum casing, a movable wall arranged therein, and a valve. The vacuum casing is mostly of tubular design, with its two end walls serving as fastenings. The brake booster conventionally has its one end wall where the valve is provided fastened to the automotive vehicle's splashboard, while the master brake cylinder is fastened to the other end wall of the brake booster. A movable wall divides the interior of the vaccum casing into a vacuum chamber in which a constant pressure prevails and a working chamber.
The pressure in the working chamber is variable dependent upon the valve which is actuated by the brake pedal. Like the pedal force which acts on a push rod via the valve, the force acting on the movable wall in the presence of a pressure differential is transmitted to the master brake cylinder via the push rod which is axially disposed in the vacuum casing.
The vacuum casing is conventionally of sheet steel. As a result of the pressure differential between the atmospheric pressure prevailing outside the vacuum casing and the vacuum, forces will be produced causing a deformation of the vacuum casing. In addition to the forces caused by the pressure differential, further forces transmitted via the push rod occur during brake actuation, i.e., the brake pedal force and the boosted force generated by the brake booster. The sum of the forces acts on the piston of the master cylinder, generating in the master cylinder a reaction force of the same magnitude, yet acting in the opposite direction.
The portion of the reaction force coming from the pedal force is transferred from the master cylinder via the sheet parts of the vacuum casing to the splashboard where the brake pedal is also fastened. The portion of the reaction force coming from the booster force ends at the vacuum casing. The reaction force which takes its course via the vacuum casing causes an extension of the vacuum casing in an axial direction during brake actuation, resulting in displacement of the master cylinder and consequently in an additional lost travel of the brake pedal. Because of the brake lever transmission ratio, the extension of the vacuum casing will be multiplied when transmitted to the brake pedal.
It has been proposed in the U.S. copending application Ser. No. 061,113, filed July 26, 1979, to provide in the vacuum casing a tube extending in an axial direction and connected to the end walls of the vacuum casing with the movable wall sealed relative to, and slidably arranged on, the tube.